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To lower the introduction and maintenance costs of autonomous power supplies for driving Internet-of-things (IoT) devices, we have developed low-cost Fe-Al-Si-based thermoelectric (FAST) materials and power generation modules. Our development approach combines computational science, experiments, mapping measurements, and machine learning (ML). FAST materials have a good balance of mechanical properties and excellent chemical stability, superior to that of conventional Bi-Te-based materials. However, it remains challenging to enhance the power factor (PF) and lower the thermal conductivity of FAST materials to develop reliable power generation devices. This forum paper describes the current status of materials development based on experiments and ML with limited data, together with power generation module fabrication related to FAST materials with a view to commercialization. Combining bulk combinatorial methods with diffusion couple and mapping measurements could accelerate the search to enhance PF for FAST materials. We report that ML prediction is a powerful tool for finding unexpected off-stoichiometric compositions of the Fe-Al-Si system and dopant concentrations of a fourth element to enhance the PF, i.e., Co substitution for Fe atoms in FAST materials.

To develop an autonomous power generation technology to support an internet-of-things (IoT) society, we proposed low-cost and nontoxic Fe-Al-Si-based thermoelectric (FAST) materials consisting of an Fe3Al2Si3 phase. Because of the cost-effectiveness and easy disposal of FAST materials, they are attractive for autonomous power supplies to drive IoT sensors and devices. While bismuth-tellurium-based thermoelectric power generation modules have been commercialized, the discovery of FAST materials opens an additional route to generate power from waste heat at room temperature under a small temperature difference, which will expand the diversity of applications of thermoelectric power generation modules. This paper reports the thermoelectric properties of FAST materials synthesized by conventional laboratory-scale synthesis and mass production processes, enhancement of power factor less than 600 K through homogenization and removal of metallic precipitations, development of thermoelectric power generation modules, and the results of power generation tests. The operation of temperature/humidity sensors and wireless transmission by Bluetooth low energy communication using FAST materials-based modules under a small temperature difference at room temperature was demonstrated.

The activated reactive consolidation method was implemented to enhance the thermoelectric properties of n-type Mg2Si binary components. This method consists of the preparation of a nanocomposite of...

The growth of Si precipitates in Mg2Si matrix under an external stress has been simulated by the phase-field method. The temporal evolution of a microstructure was determined by solving the time-dependent Cahn-Allen equations for the concentration and Ginzburg-Landau equations for the order parameters for precipitates. We observed that the external stress accelerates the Si precipitate growth, which is mainly due to reduction of the free energy of the Si precipitates under an external stress. Under a tensile stress, the Si precipitates preferentially align along the stress direction. Increasing the tensile stress accelerates the growth and decreases the aspect ratio of the Si precipitates. Finally, the Si precipitates aggregate to form a lath structure for all cases. Under a compressive stress, the growth rate is larger and the aspect ratio of the Si precipitates is smaller than those in the case of tensile stress at the same absolute value of the applied stress. We found the obtained microstructure contains uniformly dispersed Si lenticular precipitates in the Mg2Si matrix, which is suitable for thermoelectric materials because the phonons are frequently scattered at the interface between the Si precipitates and the Mg2Si matrix. Our present simulation results suggest that thermoelectric properties of this type of material will be significantly improved compared to the same material without stress.

A compositionally graded alloy in the PbTe–Sb2Te3–Ag2Te system was fabricated by the Bridgeman method. In the sample obtained for analysis, the length of the graded area was of the order of several tens of nanometers. Its thermal effusivity was measured with a thermal microscope, which required sputtering of a Mo film on the surface of the sample. The measured values of thermal effusivity depended on the thickness of the Mo film. However, it was difficult to sputter the Mo film uniformly in the wide area. Even though a relatively large target was used in the sputtering apparatus on the laboratory scale, the thickness of the Mo film just under the center of the target was larger than that in other areas. In this study, the measured thickness of the Mo film was calibrated with a high degree of accuracy against the reference glass slide. The thermal effusivity of the PbTe–Sb2Te3–Ag2Te alloy was in the range of 400–800 Ws0.5 m−2 K−1.

The growth process of pores in aluminum during directional solidification was simulated by the multi-phase field method. First, the effect of solid-liquid interface moving speed (V) and temperature gradient (G) on pore size was investigated. We found that the simulation results coincided well with the actual experimental results. Second, V was changed during the formation of the lotus metal under constant G and the appropriate condition was determined for the pore to extend steadily. We confirmed that by controlling V to be negatively proportional to the pore growth rate, the pore could be extended while its width was kept constant.

A mechanism for rapid, single-step synthesis and fabrication of intermetallic compounds having brittle–ductile ingredients at low temperature is suggested. Employing this mechanism, highly pure, dense Mg2Si pellets were obtained using silicon nanoparticles and micron-sized magnesium particles as starting materials. Silicon nanoparticles chop magnesium particles and produce highly-activated mixture through a planetary ball-milling. Magnesium/silicon interface increases by increasing dispersity of silicon nano-particles in magnesium. Consequently, final product was prepared only by consolidation at 300∘C for 5[Formula: see text]min without any post-treatment. The pure phase of Mg2Si with grain sizes about 200[Formula: see text]nm and with no evidence of presence of MgO was achieved. The Seebeck coefficient, electrical and thermal conductivity were measured to be [Formula: see text]220 [Formula: see text]VK[Formula: see text], 72 [Formula: see text][Formula: see text]cm[Formula: see text] and 3.48[Formula: see text]Wm[Formula: see text][Formula: see text]K[Formula: see text] at 823[Formula: see text]K for nondoped Mg2Si, indicating that this method does not degrade the properties of the product, despite the fact that fabrication is performed rapidly and at low temperature.

Microstructure control in functional materials draws from a historical reserve rich in established theory and experimental observation of metallurgy. Methods such as rapid solidification, eutectoid reaction, and nucleation and growth precipitation have all proven to be effective means to produce microstructure relevant for a wide array of applications. Here, the available parameters to control structure morphology, size, and spacing are discussed using thermoelectric composites as an example. Moreover, exploiting different aspects of a material system's phase diagram enables a controlled introduction of nanostructures. While much of this discussion is pertinent to the rapidly developing field of thermal conductivity control in thermoelectric composites, these techniques can be applied to a variety of other material systems where their use may lead to novel electrical, optical, as well as thermal properties of semiconductors and insulators as it has in the past for the mechanical properties of metals.

Pb-alloyed AgSbTe_2 (Pb_xAg_(20)Sb_(30−x)Te_(50) (x = 3, 4, 5 and 6)) composites were synthesized using a modified Bridgman method with a graphite mold to form plate-like samples. The Bridgman-grown specimens were dense, with few solidification cavities, and were sufficiently mechanically robust for a variety of electronic/thermal transport measurements. Inhomogeneity was found on the grain boundary, and was embedded with the nanoprecipitates of δ-Sb_2Te with a feature size of 100 nm of the 5 at.% Pb and 6 at.% Pb specimens. A combined effect of alloying, inhomogeneity and nanoprecipitates leads to a low thermal conductivity of 0.3–0.4 W m^(−1) K^(−1), which approaches the theoretical minimum thermal conductivity of the amorphous material (κ_(min) ∼ 0.36 W m^(−1) K^(−1)). A peak of the zT value, ranging from 0.7 to 0.8, is achieved at 425 K. Further annealing at 673 K increases the grain size and causes a reduction in the value of the zT peak to 0.4.

The microstructure of Bi_(90)Sb_(10) composites with separate additions of Ag and As was studied, and measurements of the room temperature transport properties were made to see the effects of hot pressing with and without precipitates. The most successful nanostructured Bi_(90)Sb_(10) samples were those containing As when annealed for durations of 3 days or less near the solidus temperature. The transport measurements showing similar electrical resistivity and decreased thermal conductivity with the addition of precipitates at room temperature demonstrates the potential of this bulk nanostructuring and synthesis approach in this material system. With further study, this work could be helpful in creating similar complex nanostructured systems which have the potential to yield high-efficiency thermoelectric materials for low temperature applications.

Nucleation and growth precipitation experiments were conducted on samples of In-alloyed Bi_2Te_3 with a starting composition of Bi_(1.65)In_(0.35)Te_(3) (7 at.% In) to investigate the phase diagram of the Bi_2Te_3–In_2Te_3 system. Temperature-dependent equilibrium solubilities were determined at 500 and 450 °C and also the maximum solubility of In in Bi_2Te_3 was verified by unidirectional solidification using the Bridgman method. The In_2Te_3 structures formed during precipitation are either plates or elongated ribbons with thicknesses of the order of hundreds of nanometers with a heavily temperature-dependent spacing distribution. All in-grain precipitation occurs along the {0 0 0 1} family of planes in Bi_2Te_3. The orientation relationship between Bi_2Te_3–In_2Te_3 was found to be {0 0 0 1}Bi_2Te_3 || {1 1 1}In_2Te_3 and consequently 〈 1 1 2¯ 0 〉 Bi_2Te_3 || 〈1 1 0〉 In_2Te_3, which aligns the close-packed planes and directions. The resulting matrix is expected to be lightly doped and may be suitable for microstructure engineering for near room temperature thermoelectric applications.

A new route to self-assembled nanocomposite thermoelectric materials is proposed. High-energy mechanical alloying brings materials into a nonequilibrium intermediate state, such as a solid solution with an extended solubility. The large driving force for the transformation to the equilibrium state leads to nanometer-scale microstructure formation, which is ideal for reducing lattice thermal conductivity.

Regular arrays of nano-wire microstructures containing Ag_2Te rods with diameter

Understanding the structure and formation mechanisms of interfaces between different telluride phases is important to the development of thermoelectric nanocomposites. Here, we investigate the interfacial structure of tetradymite precipitates in a rocksalt telluride matrix, focusing in particular on precipitates of Sb_2Te_3 in PbTe. Using high-resolution transmission electron microscopy, we investigate the structure and arrangement of interfacial disconnections—i.e. interfacial steps possessing dislocation character—observed in this system. Our analyses provide insight concerning the roles of these defects in accommodating the large interfacial misfit (6.7%) in this system and in mediating the transformation from the rocksalt to the tetradymite structure. Our observations also suggest how such interfacial disconnections could arise through the dissociation of crystal lattice dislocations that accommodate the misfit on initially flat segments of the interface.

The solvus lines of the PbTe and Ag_2Te phases in the pseudo-binary PbTe–Ag_2Te system have been determined using diffusion couples and unidirectional solidification by the Bridgman method. The solubilities of both Ag_2Te in PbTe and PbTe in Ag_2Te decrease with decrease in temperature. For the former, this change is from 14.9 at% Ag (694 °C) to 0.5 at% Ag (375 °C), while for the latter it is from 12.4 at% Pb (650 °C) to 3.1 at% Pb (375 °C). The decrease in solubilities leads to the formation of precipitates of Ag2Te in PbTe and PbTe in Ag_2Te. In particular, fast atomic diffusion in Ag_2Te results in the precipitation of PbTe even in quenched samples. From the temperature dependence of these solubilities, heats of solution have been determined. In the diffusion couple, the phase boundary moves toward PbTe. In the region between the phase boundary and the initial interface, PbTe transforms to β-Ag_2Te (cubic) retaining the cube-on-cube orientation relationship.

The number density and area per unit volume of Sb_2Te_3 Widmanstatten plates in thermoelectric PbTe were controlled through two types of heat treatments of (PbTe)_(1−x)-(Sb_2Te_3)_x, where x = 0.04 and 0.06: isothermal annealing at various temperatures and cooling from a solid-solution regime to a two-phase region with various rates. The microstructure was quantified by image analysis of scanning electron micrographs and Rietveld refinements of X-ray diffraction profiles. Isothermal annealing of (PbTe)_(0.94)-(Sb_2Te_3)_(0.06), results in increasing number density and area per volume of precipitates with decreasing temperature. In controlled cooling rate experiments, faster cooling rates or smaller x result in higher number density and area per volume. These trends are discussed using phase transformation theories. Overall the number density and area per volume of precipitates were controlled in the ranges from 0.4 to 44 μm^(−3) and from 0.5 to 1.8 μm^(−1), respectively. Isothermal annealing was performed for time periods from 10 to 166 h at 723 K to check the stability of the microstructure at the (PbTe)_(0.94)-(Sb_2Te_3)_(0.06) composition. While the Boyd and Nicholson model of the Greenwood–Lifshitz–Slyozov–Wagner theory for the average diameter of plates gives a reasonable value for peripheral interfacial energy, the time dependence was found to decelerate more than the t^(1/3) rule. It has also been found that the coarsening mechanism involves the elongation of plates.

A unidirectional solidification experiment by Bridgman method has been performed for the Pb_(14)Bi_(28.8)Te_(57.2) composition, which lies on the pseudo-binary PbTe–Bi_2Te_3 system, resulting in the formation of Widmanstatten precipitates of a ternary compound, most likely with the structure of PbBi_2Te_4 in the PbTe matrix. The formation of the precipitates is caused by the decrease of bismuth solubility in the PbTe phase with decreasing temperature. The PbTe-rich part of the PbTe–Bi_2Te_3 phase diagram was investigated from the compositional variations in the unidirectionally solidified sample and the diffusion couples. This proved that the solubility decreases with decreasing temperature: 15.6 ± 0.9 (583 °C) to 62^(+21)_(−17) (450 °C) at.% Bi. The orientation relationship between the matrix and precipitates has been examined by electron backscatter diffraction technique; precipitation occurs on {111} habit planes in PbTe with orientation relationship (0001)_(precipitate)//{111}_(PbTe) and _(precipitate)// _(PbTe). The thermoelectric properties in PbTe with Widmanstatten precipitates as examined by the scanning Seebeck probe method is –46 ± 2 μVK^(−1).

In situ high-temperature powder X-ray diffraction experiments have been performed on performed on Pb10.5Sb6Te57.9 using using a synchrotron X-ray source to observe the eutectoid reaction from Pb2Sb6Te11 to PbTe and Sb2Te3. Molar fractions of the constituent phases and fraction transformed were obtained as functions of time. The experimental fraction transformed vs. time curve were well fitted by the Kolmogorov–Johnson–Mehl–Avrami equation with an Avrami index n = 1.4. The small n value suggests that the growth of the new phases PbTe and Sb2Te3 is one-dimensional.

Single crystals of SrAl_2Si_2 were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl_2Si_2 is isostructural to CaAl2Si2 and crystallizes in the hexagonal space group P–3m1 (90 K, a=4.1834 (2), c=7.4104 (2) A, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at 1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of 7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from −78 μV K^(−1) at room temperature to −34 μV K^(−1) at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm^(−1) K, and contributes to the overall low zT for this compound.

Unidirectional solidification experiments have been performed with a thermoelectric alloy with a starting composition of Pb_(10.5)Sb_(31.6)Te_(57.9) in the pseudobinary PbTe–Sb_2Te_3 system. The bottom of the resulting rod consists of PbTe phase with Widmanstatten precipitates of Sb_2Te_3. The precipitation is due to a decrease in the solubility of Sb_2Te_3 with temperature: the solubility at 450 °C was determined to be 2.1 ± 0.2 at.% Sb. The average thickness of plates was estimated to be ~100 nm. The spacings between neighboring plates has a distribution in the 200–3000 nm range, peaking around 900 nm. The habit planes of precipitation are of the {111}PbTe family. An orientation relationship of (0001)Sb_2Te_3//{111}PbTe and Sb_2Te_3// PbTe was found with a maximum misorientation of 15°. The Seebeck coefficient after annealing at 450 °C was −50 ± 10 μV °C^−1 at room temperature. To improve the thermoelectric properties, tuning of the carrier concentration would be necessary.

Single crystals of boron-doped Ba8Al14Si31 clathrate I phase were prepared using Al flux growth. The structure and elemental composition of the samples were characterized by single-crystal and powder X-ray diffraction; elemental analysis; and multinuclear (27)Al, (11)B, and (29)Si solid-state NMR. The samples' compositions of Ba8B0.17Al14Si31, Ba8B0.19Al15Si31, and Ba8B0.32Al14Si31 were consistent with the framework-deficient clathrate I structure Ba8Al(x)Si(42-3/4x)cube(4-1/4x) (x = 14, cube = lattice defect). Solid-state NMR provides further evidence for boron doped into the framework structure. Temperature-dependent resistivity indicates metallic behavior, and the negative Seebeck coefficient indicates that transport processes are dominated by electrons. Thermal conductivity is low, but not significantly lower than that observed in the undoped Ba8Al14Si31 prepared in the same manner.

Rare-earth transition metal compounds Yb_(14)Mn_(1−x)Zn_xSb_(11), isostructural with Ca_(14)AlSb_(11), have been prepared using a metal flux growth technique for thermoelectric property measurements (with x = 0.0, 0.2, 0.3, 0.4, 0.7, 0.9, and 1.0). Single-crystal X-ray diffraction and electron microprobe analysis data indicate the successful synthesis of a solid-solution for the Yb_(14)Mn_(1−x)Zn_xSb_(11) structure type for 0 0.4. High-temperature (298 K–1275 K) measurements of the Seebeck coefficient, resistivity, and thermal conductivity were performed on hot-pressed, polycrystalline samples. As the concentration of Zn increases in Yb_(14)Mn_(1−x)Zn_xSb_(11), the Seebeck coefficient remains unchanged for 0 ≤ x ≤ 0.7 indicating that the free carrier concentration has remained unchanged. However, as the nonmagnetic Zn^(2+) ions replace the magnetic Mn^(2+) ions, the spin disorder scattering is reduced, lowering the resistivity. Replacing the magnetic Mn^(2+) with non magnetic Zn^(2+) provides an independent means to lower resistivity without deleterious effects to the Seebeck values or thermal conduction. Alloying the Mn site with Zn reduces the lattice thermal conductivity at low temperatures but has negligible impact at high temperatures. The reduction of spin disorder scattering leads to an ∼10% improvement over Yb_(14)MnSb_(11), revealing a maximum thermoelectric figure of merit (zT) of ∼1.1 at 1275 K for Yb_(14)Mn_(0.6)Zn_(0.4)Sb_(11).

Zintl phase compounds with large unit cells and complex anionic structures such as Yb11Sb10 hold potential for being good thermoelectric materials. Single crystals of Ge-doped Yb11Sb10 were synthesized using a molten Sn-flux technique. Single crystal X-ray diffraction data were obtained and resulted in a composition of Yb11Sb9.3Ge0.5 which was verified by microprobe. Yb11Sb9.3Ge0.5 is isostructural to Ho11Ge10, crystallizing in a body-centered, tetragonal unit cell, space group I4/mmm, with Z = 4. The unit cell parameters of Yb11Sb9.3Ge0.5 are a = 11.8813(4), c = 17.1276(13) A with a volume of 2417.8(2) A3. These parameters correlate well with the structural refinement of previously published Yb11Sb10. The structure consists of 16 isolated Sb3− anions, 8 dumbbells, 2 square planar rings and 44 Yb2+ cations. The Ge, doped in at 28 % occupancy, was found to be site specific, residing on the 2 square planar rings. Single crystal X-ray diffraction is most consistent with the site that makes up the square ring being less than fully occupied. The doped compound is additionally characterized by X-ray powder diffraction, differential scanning calorimetry and thermogravimetry. High temperature (300–1200 K) thermoelectric properties show that the doped compound has extremely low thermal conductivity (10–30 mW/cmK), lower than that of Yb11Sb10. Temperature dependent resistivity is consistent with a heavily doped semiconductor. Yb11Sb9.3Ge0.5 shows p-type behavior increasing from ∼22 μV/K at room temperature to ∼31 μV/K at 1140 K. The low value and the temperature dependence of the Seebeck coefficient suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.

Motivated by reports of exceptionally high zT > 2 in thin film superlattices or “quantum well” materials with nanometer sized features, we have undertaken a study of composite materials with nanoscale features that promise to provide similar structures in bulk material. Nanometer scale layers of PbTe and Sb2Te3 with periodicities of 180 nm to 950 nm form when quenched eutectic PbTe-Sb2Te3 melt, crystallizing as Pb2Sb6Te11, subsequently annealed. The lamellar spacing depends on the temperature and time of the anneal. The mechanism for the development of the nanostructures is probed by examining the fraction of material transformed as a function of anneal time. Preliminary analysis of the shape factor exponent reveals that the transformation to the nanostructured lamellae bears similarities to the thickening of very large plates. The coarsening of the lamellar spacing is also examined as a function of time and temperature.

Lotus-type porous Ni- (15, 28 and 31) at.% Al alloys whose long cylindrical pores are aligned in one direction were fabricated by continuous zone melting technique under high-pressure gas of hydrogen of 2.5 MPa. A part of 5-10 mm in length of the rod in the vicinity of the coil was melted by high frequency induction heating, and was moved downwards by electric motors at a constant velocity of 330 μms-1 to 500 μms-1 for unidirectional solidification. The pores are formed as a result of precipitation from the supersaturated hydrogen gas when the liquid metals dissolved with gas atoms is solidified. The porosity and the pore size decrease with increasing aluminum content. An increase of solidification velocity from 330 μms-1 to 500 μms-1 leads to a decrease of pore diameter and an increase of pore number in the porous Ni-28at%Al.

The effect of pore growth direction on the weld fusion zone profile and the mechanical property of lotus-type porous magnesium joints were investigated by laser welding through experiments and numerical simulations. The dependencies of the pore growth direction on the weld fusion zone profile were evaluated on the basis of the temperature distribution calculated using the ABAQUS FE code with user subroutines. The calculated weld fusion zone profiles were in good agreement with the experimental results, thereby suggesting that thermal conductivity affects weld fusion zone profiles significantly. Moreover, the effect of pore growth direction on the joint strength of lotus-type porous magnesium was investigated. The anisotropy of the tensile strength of the lotus-type porous magnesium with regard to the pore growth direction was observed. The joints with pores parallel and perpendicular to the tensile direction were fractured at the fusion boundary and the base metal, respectively.

Lotus-type porous Cu-5at.%Al alloy whose elongated pores are aligned in one direction was fabricated by unidirectional solidification in pressurized hydrogen gas atmosphere. The porosity of the Cu-Al alloy was higher than that of pure copper under the same fabrication conditions because of difference in hydrogen solubility. The pore structure was not round shape because the directional pore growth was interrupted with dendrite arms formed during the solidification; the pores grew to detour the obstacle of the dendrite arms.

The effects of composition and cooling rate on the microstructures of alloys in the pseudo-binary PbTe–Sb 2 Te 3 system were investigated as a first step towards the design of nanostructured materials with enhanced thermoelectric properties. Liquid alloys of three different compositions were cooled in three distinct ways: water quenching, air cooling and furnace cooling. The resultant structures and phases were examined by electron microscopy, electron microprobe chemical analysis and electron backscatter diffraction. The compound Pb 2 Sb 6 Te 11 precipitated as a metastable phase (in conjunction with PbTe and/or Sb 2 Te 3 ) under all conditions. Furthermore, whereas PbTe exhibited dendritic morphology, Sb 2 Te 3 and Pb 2 Sb 6 Te 11 crystallized as lamellar platelets with preferred (0 0 1) orientation. The range of cooling rates was from ∼1 to 26 K/s, while the characteristic microstructural feature size ranged from 10 to 35 μm for dendrites, and from 15 to 50 μm for lamella. The prospects for achieving nanoscale structure are discussed.

Utilizing the decomposition of metastable Pb2Sb6Te11 into PbTe and Sb2Te3, we produced a layered (lamellar) microstructure of PbTe and Sb2Te3 in which the interlamellar spacing can be controlled by the temperature and time of the decomposition process. Adjacent PbTe and Sb2Te3 lamellae are crystallographically oriented, indicating high-quality epitaxy-like interfaces. Average lamellar spacings as small as 180 nm are observed, corresponding to a PbTe layer thickness of 40 nm. These nanoscale multilayers, formed by bulk processing, resemble thin-film superlattice thermoelectric materials, which have shown exceptionally high thermoelectric efficiency.

The diffusion coefficients of 44Ti, 63Ni and 59Fe in γ-TiAl single crystals have been measured by ion-beam sputter-sectioning technique, while those of In have been measured using ion implantation technique and secondary ion mass spectroscopy (SIMS) in order to clarify the diffusion anisotropy: the diffusion perpendicular and parallel to the [001] axis. The diffusion of Ti and In perpendicular to the [001] axis is faster than that parallel to the [001] axis. However, the diffusion anisotropies of Fe and Ni show opposite trend to those of Ti and In, namely the diffusion parallel to the [001] axis is faster than that perpendicular to the axis. The predominant process of diffusion perpendicular to the [001] axis has been discussed from a viewpoint of activation energy using the expression of the diffusion coefficients in L10-ordered alloys.

The diffusion coefficient of In in γ-TiAl single crystals has been measured using the ion implantation technique and secondary ion mass spectrometry in order to clarify the diffusion anisotropy: the diffusion perpendicular and parallel to the [0 0 1] axis. The diffusion of In perpendicular to the [0 0 1] axis is faster than that parallel to the [0 0 1] axis. Such diffusion anisotropy is similar to that of Ti previously investigated by our group. The diffusion coefficients of Fe and Ni have also been measured. The diffusion anisotropies of Fe and Ni show an opposite trend to those of In and Ti, namely the diffusion parallel to the [0 0 1] axis is faster than that perpendicular to this axis. The predominant process of diffusion perpendicular to the [0 0 1] axis is discussed from the viewpoint of activation energy using the expressions of the diffusion coefficients in L1 0 -ordered alloys.

Lotus-type porous iron and stainless steel (SUS304L) whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized hydrogen or nitrogen gas atmosphere. Pores are formed as a result of precipitation from the supersaturated gases when the liquid metal dissolved with gas atoms is solidified. The ultimate tensile and yield strengths of the porous iron produced in nitrogen atmosphere are about two times higher than in a hydrogen atmosphere. Such superior strength is attributed to solid-solution hardening due to solute nitrogen atoms in iron matrix.

The tracer diffusion coefficients of Fe and Pd have been measured in FePt and FePt3 by standard radiotracer experiments. In FePt3 the diffusion coefficients are lower than extrapolations of those in the high-temperature disordered state, exhibiting curved Arrhenius plots below the transformation temperature. The diffusion of Pd is faster than that of Fe, and noticeable dependence on composition is found for the latter. In the L10 ordered FePt, the diffusion of Fe has been verified to be slower in the direction of the tetragonal axis, but the case of Pd appears more complex. The diffusion coefficients of Fe and Pd are similar in magnitude in the L10 ordered state.

The diffusion coefficient of In in TiAl has been measured using ion implantation technique and secondary ion mass spectrometry. The diffusion coefficients of Fe and Pd in FePt have been measured at two compositions by radioactive tracer method. In order to clarify diffusion anisotropy, single crystal of each alloy was used. The In diffusion perpendicular to the [001] axis is faster than that parallel to the [001] axis. Such trend is similar to Ti diffusion previously measured in our group. The diffusion of Fe in FePt perpendicular to the [001] axis is faster than that parallel to the [001] axis at each composition, while the anisotropy of the Pd diffusion is different with composition. The predominant process of the diffusion in perpendicular to the [001] has been discussed on the basis of the expressions of the diffusion coefficients.

Lotus-type porous stainless steel (SUS304L) rods were fabricated by the continuous zone melting technique under a pressurized mixed gas of hydrogen and inert gas such as argon or helium. Pores with cylindrical shape, whose growth direction is parallel to the solidification direction, are observed in the rods. The dependence of the porosity and averaged pore diameter on the partial pressure of hydrogen or the total pressure and on the transference velocity of rods was investigated. It was found that the porosity increases with increasing partial pressure of hydrogen under a constant total pressure and the pore diameter decreases with increasing transference velocity. The maximum porosity was about 60 pct under the experimental conditions in the present work. The observation of the microstructure and the measurement of the tensile strength were also carried out.

Titanium film was deposited by vapour deposition method on lotus-type porous stainless steel SUS304L fabricated by continuous zone melting technique in hydrogen atmosphere in order to obtain good biocompatibility. Titanium film of several micrometers in thickness was coated on the surface inside pores, while that of 20–30 μm was on the flat surface of the specimen. Tensile tests were carried out to evaluate the adhesive strength between the titanium layer and the substrate of stainless steel. It was found that the adhesive strength was strong enough for the use for biomedical devices.

Lotus-type porous magnesium with many unidirectional cylindrical pores was fabricated by unidirectional solidification of a melt dissolving hydrogen in a pressurized hydrogen atmosphere. The sound absorption coefficient of porous magnesium whose specimen face has many open pores was measured by the standing-wave method in the sound frequency range up to 4 kHz. The absorption coefficient increases with decreasing pore size, while it increases with porosity. Moreover, the peak value with a high absorption coefficient is shifted toward a higher sound frequency when the thickness of the porous magnesium specimen decreases. It is suggested that lotus-type porous magnesium exhibits excellent sound absorption characteristics.

Lotus-type porous (LTP) metals possess a markedly anisotropic porous structure, in which long straight pores are homogeneously aligned unidirectionally. In this paper, we first describe a procedure for the determination of reliable elastic constants of LTP metals with high porosity, and next apply the technique to LTP magnesium, and finally compare the measurement results with those from the effective-mean-field (EMF) theory that has been recently proposed by Tane and Ichitsubo [Appl. Phys. Lett. 85, 197 (2004)]. Young’s modulus in the direction parallel to the longitudinal axis of pores decreases virtually linearly with porosity p, i.e., E‖≈45.3(1−p)1.33, whereas the normal Young’s modulus E⊥, falls rapidly, i.e., E⊥≈45.3(1−p)2.64. The Mori-Tanaka mean-field theory yields values that are close to the measured elastic constants in a wide porosity range, but cannot explain this power-law behavior. In contrast, the EMF theory gives more accurate values in the high-porosity range, and can reproduce the power...

Lotus-type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus-type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod- and plate-shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus-type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus-type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.

Lotus-type porous copper is a porous medium made of copper that contains many straight pores. To effectively employ lotus-type porous copper as a heat sink, it is necessary to clarify the pore effect on the thermal conductivity of lotus copper. This article describes an experimental and analytical investigation on the effective thermal conductivities of lotus copper parallel and perpendicular to the pores. The lotus copper displayed anisotropy of the effective thermal conductivity. The effective thermal conductivity keff⊥ perpendicular to the pores was lower than that of the parallel ones keff∥ and was 40% that of lotus copper material ks with porosity e of 0.4. Experimental data for keff∥ showed good agreement with analytical results derived from the assumption that heat flow through the cross-sectional area parallel to the pore axis is proportional to (1−e). Experimental data for keff⊥ showed good agreement with the analytical results derived from the assumption of orthorhombic symmetry and with the num...

We studied temperature dependence of anisotropic elastic constants of lotus-type porous copper that consists of unidirectionally aligned pores and the matrix of oriented crystallites. All the independent elastic constants were measured from room temperature to 400 °C using an electromagnetic acoustic resonance method. All the elastic constants decrease with increase of temperature. The porosity and oriented crystallites have the influence on the temperature dependence of the elastic constants. Micromechanics theory allows calculation of the temperature dependence of the elastic constants of lotus copper from those of monocrystal copper and shows agreement with the measurements. It is concluded that micromechanics model is then valid to predict the temperature dependence of the elastic constants of the lotus-type porous metals.